This invention relates to in situ solution mining methods, and particularly to in situ solution mining methods wherein an oxygen-bearing gas is dissolved in the leach solution above ground.
In the prior art methods for leaching uranium values from underground formations in situ, an oxygenated solution of a leaching agent is introduced into the uranium ore formation through a plurality of injection wells. The oxygenated leaching agent contacts the uranium in the ore body and oxidizes the uranium so that it may be solubilized by the leach solution. The oxidation of uranium is needed to convert the uranium values from a relatively insoluble +4 valance state, UO.sub.2, to the more soluble +6 valance state, UO.sub.3. Once the leach solution has thus oxidized the uranium and the uranium has become dissolved in the leach solution, the pregnant leach solution is pumped from the ground through a plurality of production wells, and the uranium values are then recovered by known procedures such as an ion exchange process. There are many known leaching agents that can be used in such a process, such as an acid leach or an alkaline leach agent. In addition, there are many known oxidizing agents that may be introduced into the leach solution to furnish the oxidizing capability. One such oxidizing agent is hydrogen peroxide; however, hydrogen peroxide is costly and difficult to handle, and its presence may diminish the permeability of the formation through secondary precipitation reactions.
In U.S. Pat. No. 3,860,289 to Learmont, issued Jan. 14, 1975 and entitled "Process For Leaching Mineral Values From Underground Formations In Situ", there is disclosed a process for leaching mineral values from an underground formation in situ wherein the process includes introduction of oxygen to the leaching solution at a substantial depth within the injection well where the head of solution thereabove increases the solubility of oxygen in the solution. While the Learmont process does provide one method of introducing an oxidizing agent into the leach solution, it does not provide a mechanism for assuring that the oxygen has become dissolved in the solution and that the oxygen is not present in a second gaseous phase. It is important that the oxygen not be present in the underground formation in a gaseous form because the presence of oxygen gas bubbles in the ore formation is believed to cause blocking of the formation and loss of solution injectivity (i.e. reduced solution flow at constant pressure). Of course, loss of injectivity decreases the recovery rate of the uranium values from the ore body.
In U.S. Pat. No. 3,708,206 to R. A. Hard et al, issued Jan. 2, 1973 and entitled "Process For Leaching Base Elements, Such As Uranium Ore, In Situ" there is described a process for leaching base elements, such as uranium values, from an underground water saturated ore deposit containing oxidizable materials such as sulfides wherein an oxygen-bearing gas is introduced into the ore deposit prior to or simultaneously with a leach solution to oxidize the base elements within the ore deposit to a soluble state where they can be dissolved in the leach solution. The patent to Hard et al. teaches that the oxygen-bearing gas should be dispersed in a multitude of small bubbles throughout the ore body and in a manner somewhat analogous to the effervescence associated with common seltzer water. It is now believed that such a process may not recover a significant portion of the uranium values in the ore body due to the fact that the contained bubbles tend to block the formation so that the leach solution does not penetrate into certain areas of the formation wherein uranium values are located.
What is needed is a method for oxygenating a leach solution for use in in situ solution mining wherein the leach solution is oxygenated above ground so that the dissolved oxygen content may be monitored and the presence of a second phase of oxygen can be eliminated.